This application is related to Japanese Patent Application No. 2000-025445 filed on Feb. 2, 2000, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a molecular beam source and a molecular beam epitaxy apparatus, more particularly, to a molecular beam source for accommodating a molecular beam generating material therein and thermally evaporating or subliming the material to generate a molecular beam in a molecular beam epitaxy (referred to as MBE hereinafter) technique and a molecular beam epitaxy apparatus using the molecular beam source.
2. Description of Related Art
The MBE technique is a technique for generating a molecular beam by evaporating or subliming a high purity material and growing crystals on a GaAs substrate or the like in a high vacuum. It is generally used for forming semiconductor thin films of compound semiconductor devices such as semiconductor lasers and is now under research and development for further improvement.
In the MBE technique, to reduce impurities remaining in a vacuum chamber is important in production of semiconductor thin films. For this purpose, exhausters have been improved and chamber baking has been implemented in order to obtain good semiconductor thin films.
However, substances adhering to sites other than a substrate such as a shroud (a cryo panel) and the like during discharge of gasified materials and/or during crystal growth come off and fall in a molecular beam source (also referred to as xe2x80x9cmolecular beam source cellxe2x80x9d or simply xe2x80x9ccellxe2x80x9d) when liquid nitrogen is removed from the shroud. The fallen substances are re-evaporated at the next growth of crystals and result in increase of residual impurities in the vacuum chamber and a possible decline in the quality of semiconductor thin films. The re-evaporated substances may also enter a heater for heating the material mounted to a crucible of the molecular beam source cell and a lead line of a thermocouple for measuring temperature and cause troubles such as insulation failure.
To cope with this, measures have been taken such as inclining the vacuum chamber for preventing substances adhering to the shroud around the substrate and the like from falling into the cell even if they come off. Typically, the cell has a crucible in which a molecular beam generating material is fed and a heater disposed to surround the crucible almost entirely for evaporating the molecular beam generating material fed in the crucible.
With this construction, however, cells attached to upper ports of the vacuum chamber are inclined more.
Accordingly, if crucibles 901 and 902 of conventional structures shown in FIG. 11 and FIG. 12 are used, the crucibles 901 and 902 can only accommodate a decreased amount of the molecular beam generating material. As a result, the molecular beam generating material is required to be fed an increased number of times, which results in an increase in the number of maintenance operations, a decline in the availability of the MBE apparatus and an increase in production costs.
If the chamber is further inclined and the cell is mounted to the port at an angle such that an inlet opening of the cell faces toward a direction lower than a horizontal line, the crucibles 901 and 902 of the conventional structures shown in FIG. 11 and 12 cannot be used for a melt-type molecular beam generating material but can be used only for a sublime-type solid molecular beam generating material.
Published Japanese Translation of PCT International Publication for Patent Application No. HEI 11(1999)-504613 discloses a unibody crucible 903 having a negative draft orifice 904 as shown in FIG. 13. With this construction, even if the cell is horizontally laid, the melt-type molecular beam generating material can be used.
Another conventional structure for a crucible is shown in FIG. 14. The crucible 910 is comprised of a molecular beam generating material accommodating section 913, a molecular beam shape defining section 912, and a bent portion 908 formed therebetween, as shown in FIG. 14. The crucible 910 has a structure such that the molecular beam generating material accommodated in the molecular beam generating material accommodating section 913 does not face an opening 905 directly, that is, the molecular beam generating material accommodated in the molecular beam generating material accommodating section 913 cannot be seen from the opening 905.
In the above mentioned crucibles 901 and 902, if the chamber is inclined for preventing substances adhering to the shroud or the like from falling into the crucible, the amount of the molecular beam generating material that can be fed in the crucible (i.e., the capacity of the crucible) becomes smaller in a cell attached to an upper port. The number of maintenance operations for feeding the material increases, the machine operating time decreases and the production costs increase.
Also, there is a problem in that the melt-type molecular beam generating material cannot be used at a port which causes the opening of the cell to face in a downward direction.
Further, if the cell is set substantially horizontally using the crucible 903 of the conventional structure shown in FIG. 13, an evaporation area of the molecular beam generating material changes and the intensity of the molecular beam changes as the molecular beam generating material is consumed and the liquid level of the material drops. Usually, the molecular beam intensity is measured at regular intervals and compensated by adjusting the temperature of the heater. With a crucible having a structure such that the evaporation area of the molecular beam generating material is liable to change, the measurement and compensation of the molecular beam intensity must be carried out more often, which results in a decrease in the availability of the apparatus and an increase in the production costs.
Further, if the crucible 910 having the structure shown in FIG. 14 is used and the molecular beam shape defining section 912 is positioned substantially horizontally, the molecular beam intensity does not change owing to a drop in the liquid level of the molecular beam generating material, but, in order to increase the feed amount of the molecular beam generating material, the bent portion 908 of the crucible 910 need to be located at a position farther from a substrate than a cell port flange of the vacuum chamber so as to avoid contact with an outer surface of the vacuum chamber. Accordingly, if the opening 905 of the crucible 910 is set at the same position as an opening of a conventional cell, the distance from a bent portion 908 of the crucible 910 to the opening 905 becomes longer and a thinner molecular beam is emitted from the opening 905 of the crucible 910. For this reason, the crucible 910 of the above-described structure is suitable for laboratory-scale MBE apparatuses for small substrates, while it cannot provide a uniform film thickness in industrial-scale MBE apparatus for large substrates.
In view of the above-described circumstances, an object of the present invention is to provide a molecular beam source which is capable of accommodating a large amount of the molecular beam generating material and also provides a uniform film thickness on a large substrate and a molecular beam epitaxy apparatus using the molecular beam source.
The present invention provides a molecular beam source comprising a crucible having an opening, and a heater mounted to the crucible for evaporating by heating a molecular beam generating material accommodated in the crucible to emit a molecular beam from the opening, wherein the crucible has an accommodating section for accommodating the molecular beam generating material; a bent portion provided between the opening and the accommodating section so that the molecular beam generating material accommodated in the accommodating section does not face the opening directly; and a narrowed portion between the bent portion and the opening.
The molecular beam source according to the present invention is provided with the crucible which is so bent that the molecular beam generating material accommodated therein does not face the opening directly, i.e., that the molecular beam generating material accommodated in a bottom portion of the crucible is not seen from the opening, and which has, between the bent portion and the opening, the narrowed portion whose cross-sectional area is reduced. Therefore, the molecular beam source has the following advantages: It is applicable for a melt-type molecular beam generating material; the molecular beam generating material is prevented from scattering toward the opening even if it boils suddenly; and it is ensured that the molecular beam generating material is fed in a sufficient amount and also the molecular beam emitted from the opening of the crucible can diffuse radially, and therefore, a uniform film thickness can be obtained on a large substrate.
In the molecular beam source of the present invention, if the crucible is comprised of an accommodating section from the bent portion to the bottom portion in which the molecular beam generating material is accommodated and a molecular beam shape defining section from the bent portion to the opening of the crucible, the narrowed portion may be formed at least at one site in the molecular beam shape defining section.
As a particular shape of the molecular beam source of the present invention, the crucible may be in a tubular shape having an inside wall parallel to the direction of its axis throughout its length, preferably in a cylindrical shape having a constant inner diameter, the central axis of the accommodating section and that of the molecular beam shape defining section are in the same plane, and these central axes form an angle of 30xc2x0 to 150xc2x0.
The accommodating section is preferably in such a shape that the evaporation area of the molecular beam generating material does not change if the amount of the molecular beam generating material changes.
The narrowed portion preferably has an inner diameter which is {fraction (1/20)} to xc2xd of the maximum inner diameter from the bent portion to the opening.
If the shape from the narrowed portion to the opening is substantially conic, molecular beams can be generated from the central axis of the molecular beam shape defining section regularly in radial directions.
If the crucible is constructed to have at least two components which are connectable at a position nearer to the bent portion than to the narrowed portion, the crucible can be divided, for example, into the accommodating section formed of a tube having a constant diameter and the molecular beam shape defining section having the narrowed portion. Accordingly, the crucible may have a complicated shape and the degree of freedom in determining the shape of the crucible increases. Also the molecular beam generating material is fed in the accommodating section more easily.
If the components of the crucible are constructed to be connected to each other by a friction fit, it facilitates the separation and connection of the components at a joint.
In the present invention, the molecular beam source may preferably be provided with at least two sets of heaters whose temperatures are separately controlled. Further, if a heater disposed at the molecular beam shape defining section is so arranged that a part of the heater disposed from the opening to the narrowed portion of the crucible is denser than a part of the heater disposed in other region of the molecular beam shape defining section, the material can be prevented from adhering to the opening of the crucible where the temperature would otherwise be liable to drop.
In another aspect of the present invention, there is provided a molecular beam epitaxy apparatus comprising a molecular beam source; a vacuum chamber for supporting the molecular beam source; a shroud provided along an inner wall of the vacuum chamber; and a substrate holder provided in the shroud opposedly to the opening of the crucible of the molecular beam
The molecular beam epitaxy apparatus of the present invention may be provided with a plurality of molecular beam sources.
These and other objects of the resent application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.